did you try w/o gas-tube? still hangs up?
is your saiga converted? is gas block straight on?
does it hang up without mag inserted?
are rails straight and uniform (both top and bottom rails)?
the more i think about it the more it seems like a gas-tube fault. AK BCG is sloppy and wobbly it's basically wants to be guided by gas tube into the gas block. try using original gas tube and see if trouble goes away, or at least we can rule out if it's specifically ultimak-related issue or gas-tube issue in general. if still hangs up with original gas-tube AND rifle undergone conversion with surplus parts then it's possible gas block might be canted.
another thought here: Saigas are essentially AK-74M. which means gas tube mounting is different than AK-47/AKM gas-tubes. It should have a spring and a lip at the bottom and as you push gas tube down at the rear sight block when re-assembling it *should* go-down with some effort then snap-in and move back slightly. this spring was added to maintain tight fit of the gas tube as well as ensure it stays latched unto the bottom of the rear sight block. Ultimak is lacking these provisions replacing it with it's bolt-on barrel bands. so i can see how it might not be doing what OEM tube should. personally i don't own Ultimak but looking at one in the picture i question whether that could be the issue responsible.
go ahead and give original tube a try and see if problem persists. also add more pics. hard to see what's going on with only such a limited area of the rifle in the view.